1. Field of the Invention
The present invention relates to a slide fastener element to be manufactured by punching out a metallic flat type wire rod, and a method for manufacturing the element.
2. Description of the Related Art
A metallic element of a slide fastener, the element comprising a coupling head and right and left leg portions extending separately as two branches from the head on a same plane, is generally manufactured according to the following two typical manufacturing methods. According to one of these methods, a long wire rod having a Y-shaped section is rolled out through multiple stages, and then this wire rod is cut out successively in a longitudinal direction of the wire rod in a desired thickness so as to obtain a Y-shaped element material. By deforming a portion of that material corresponding to a coupling head locally under a pressure, the engagement portion is formed and elements are produced. The elements produced in this way are implanted successively at a constant pitch on a side edge of a long fastener tape carried separately so as to manufacture a fastener chain continuously.
According to the other method, a flat type wire rod is punched out into the shape of an element by using a die or a punch successively, and it is deformed plastically under a pressure so as to form a coupling head, thereby producing elements individually. After the elements manufactured in this way are subjected to barrel polishing or chemical polishing, they are implanted successively along a side edge of a fastener tape at a constant pitch so as to produce a continuous fastener chain. In the element manufactured by the latter method, the shape in appearance and structure of the coupling head of the element are particularly wide-ranging, since the die and the punch can be formed into diversified shapes. In the simplest shape of the coupling head, one side of the central portion of the coupling head defined by combination of a trapezoidal shape and a rectangular shape is recessed while the other side is constituted of an engagement portion projecting in a mountainous shape. On the other hand, other typical structures have been disclosed in, for example, Japanese Patent Application Publication No. 59-10858 (patent document 1), Japanese Patent Application Publication No. 59-10859 (patent document 2), Japanese Utility Model Application Laid-Open No. 1-80012 (patent document 3) and the like.
As for the typical shape and structure of an element obtained by punching out a flat type wire rod, which is disclosed in these patent documents 1 to 3, the element comprises a coupling head 2 and right and left leg portions 4 as shown in FIG. 1. The coupling head 2 includes a flat plate portion 8 having a thickness ⅓ of that of the leg portions 4, swollen portions 3, which are engagement portions swollen on front and rear faces substantially in a center of the flat plate portion, and fitting concave portions 6, which are formed in a flat face portion extending on the same plane as the leg portions of the flat plate portion 8, such that each of the fitting concave portions surrounds a side face on a leg portion side of the swollen portion 3 with a predetermined gap. A part of the swollen portion 3 of a mating element is fitted in between the fitting concave portion 6 and the swollen portion 3, so that the elements are engaged with each other. In addition, the element further comprises nipping portions 7 which are provided at front ends of the right and left leg portions 4 and extend so as to approach each other.
Meanwhile, in the metallic element in which the aforementioned metallic wire rod having a Y-shaped section subjected to being rolled out is cut and then, the engagement portion of the coupling head is molded by pressing, no abnormal quality face is formed on a processed surface of the element because an external peripheral faces of the right and left leg portions and the coupling head of the obtained element are rolled out, so that entirely uniform surface is obtained. However, as for the metallic element called an individual element which is disclosed in the patent documents 1 to 3, a sheared portion and a fracture portion are usually generated in a cut surface when the element is cut out and punched out with a punch. In the sheared portion, an extremely flat shear surface is formed and a fracture surface having a high roughness, containing fine unevenness like scratched flaw is formed in a part of the fracture portion. The metallic element called the individual element is often used in a slide fastener to be attached to a high quality product. Thus, in case of this individual element, an outside exposed surface of the element after implanted in a fastener tape needs to be finished at a high precision.
Further, this kind of the individual element is subjected to barrel polishing or chemical polishing for finishing after the element material is produced by punching out the metallic flat type wire rod by using the die and the punch as described above and at this time, the roughness of a fracture surface is often intensified by such polishing action. For the reason, after finished as a product, a difference between the shear surface and the fracture surface become evident. Thus, in a slide fastener in which such elements are implanted in the fastener tape, particularly the surface of an outside exposed portion of the element, that is, the outer side faces of right and left leg portions and the outer peripheral face of the coupling head look like not finished uniformly. This can be a fatal defect for a slide fastener attached to a high quality product.
On the other hand, to fix this kind of the metallic element to the fastener tape, a core thread portion formed on a side edge of the fastener tape is nipped by right and left leg portions and the right and left leg portions are caulked firmly. The fixing strength at this time depends on the amount of biting of the front end nipping portions of the right and left leg portions into the fastener tape and the nipping strength to the core thread portion. As long as this viewpoint is taken, the higher roughness of the inner peripheral face of each of the right and left leg portions is more preferable.